1. Field of the Invention
This invention pertains to radioactive tracers and their use in wells. More particularly, in one aspect, encapsulated particles are provided which are used for radioactive tracing. In another aspect, encapsulated radioactive particles are added to fluid injected into a well to determine the location of the traced fluid outside the casing of the well by radioactive logging of the well.
2. Description of Related Art
A variety of treating fluids are pumped into wells. Such treating fluids include acid and salt solutions, surfactant solutions, mineral-dissolving solutions, organic resins and cement slurries. In most instances, these fluids are pumped down the tubing or casing of a well and exit through a plurality of perforations in the casing. Various techniques are often used in an attempt to divert the treating fluid such that it flows uniformly through the spaced-apart perforations in the casing of the well. Such diverting techniques include slugs of gelled material, slightly soluble solids, ball sealers and mechanical devices set inside the casing. In any case, it is desirable to know where the treating fluid has entered the formation outside the casing and the relative amounts of treating fluid exiting through the different perforations. In the case of injection of cement slurry into a well, if it is used in the process of squeeze cementing it is desirable to know where the slurry that has been pumped through perforations is located. In primary cementing, it is desirable to know where the cement slurry has been placed as it is pumped down the casing and up around the outside of the casing, particularly where the top of the cement is located.
The application of radioactive materials to determine the flow path or location of fluids and solid particles injected into wells has been reviewed ("Improved Evaluation Techniques for Multiple Radioactive Tracer Applications," 12th Int'l Logging Symposium of SAID, Paris, 1989; "Radioactive Tracers Improve Completion and Fracturing Practices," Pet. Engr. Int'l., March 1995). With the development in recent years of spectral logging tools for logging different gamma ray-emitting isotopes in the same well, the use of multiple radioactive tracers injected at different times or in different injection fluids or solid particles has offered significant advantages in analysis of well conditions.
The radioactive elements can be present as a soluble compound in liquid, as insoluble or slightly soluble particles of the element (or a compound of the element) suspended in the flow stream, or as a soluble or insoluble compound attached to particles or contained within particles which are suspended in the liquid. Water-soluble compounds containing radioactive iodine have been widely used for tracing flow of fluids between wells and in the vicinity of a well. U.S. Pat. No. 5,243,190 provides an example of radioactive elements incorporated within ceramic particles and used for tracing flow of particles in wells, particularly proppant particles employed in the process of hydraulic fracturing of wells. The particles used for tracing particle flow are usually made to have about the same size and density as the particles being traced. Those radioactive particles used in hydraulic fracturing of wells are from about 100 mesh to about 20 mesh (75 micrometers to 850 micrometers) in size. The radioactive compounds are present at a relatively low concentration in the particle, so that the properties of the particle are not significantly affected by the presence of the radioactive compound. The specific activity (millicuries radioactivity per volume or mass of particles) of the particles, therefore, is relatively low. For example, in the aforereferenced U.S. Pat. No. 5,243,190, the specific activity of the particles, made from ceramic materials mixed with material which can be made radioactive by neutron bombardment, was from 0.02 to 20 mlllicuries (mCi) per milliliter of particles. The density of these particles was designed to be approximately the same as the density of the non-radioactive particles when applied to tracing proppant in hydraulic fracturing operations. Ceramic particles have also been used for tracing flow of add, where the density of the particles was decreased to the range of about 1.1 to 1.5 gm per ml. ("Using Low Density Tracers to Evaluate Acid Treatment Diversion," SPE 29587, Society of Petroleum Engineers, 1995). Particles made of ceramic components are difficult and expensive to manufacture in very small particle sizes, however, and are normally limited to relatively low specific activity because the radioactive element is present at low concentration in the solid particles.
For tracing the location of treating fluids around a well, water-soluble tracers in a treating fluid have two disadvantages: they are easily displaced away from the well or back into the well and they contaminate surface and well equipment. Soluble tracers produced back into a treated well at excessive concentration after the well has been treated and placed on production can present a problem of disposal of the fluid. Contamination of surface equipment or equipment used in the treating process is particularly troublesome, as it can cause hazardous conditions for personnel and require expensive cleaning procedures.
What is needed is a highly dispersed radioactive (gamma ray emitting) material which can be easily manufactured without undue hazard or expense, has a high specific activity, will flow along with treating fluids, will leave little or no contamination of surface or downhole equipment as the treating fluids are pumped into wells and will not be produced back in production fluids at high concentration.